THE NAUTILUS. 63 



having, perhaps, no examples of circumcarinata at hand for direct 

 comparison with elrodi. 



The following differences appear on comparing the shells : With 

 the same general figure and size, P. elrodi has a wider umbilicus, 

 and viewed from above, the last whorl is wider ; the base is more 

 convex, being swollen and almost subangular around the umbilicus ; 

 the rib sculpture is coarser ; the finer spiral sculpture is much de- 

 veloped in P. elrodi, especially beneath, while E. circumcarinata 

 shows no spirals there, but only minute papilla; scattered between 

 the ribs. The shape of the apertures differs : In P. elrodi the trans- 

 verse axis does not much exceed the longitudinal, while in circum- 

 carinata it is conspicuously greater. In Dr. Stearns' species, the 

 basal lip is narrowly reflexed, while in P. elrodi it is at most merely 

 expanded a little. Finally, the apices show important differences. 



In K. circumcarinata the first H whorls appear smooth, a high 

 power showing a minute, even, criss-cross pebbly sculpture, like that 

 of E. mormonum, and the next whorl shows only very weak riblets, 

 with papillae between them ; while in P. elrodi the strong riblets be- 

 gin earlier, and there is no trace of the mormonum type of sculpture, 

 even in young removed from the uterus of the mother. 



These facts indicate, in my opinion, that the great general simi- 

 larity between the two species in question is merely superficial, and 

 probably the result of similar conditions acting upon organisms 

 originally diverse, and indeed not closely related. P. elrodi belongs 

 undoubtedly to the P. strigosa group ; while, although its soft 

 anatomy is unknown, circumcarinata will probably prove to belong 

 where Dr. Stearns placed it, near mormonum, although with present 

 knowledge it is in all probability distinct from that specifically. In 

 other words, I think the two species belong to different genera. 



It might be as well to say here that the strigosa group of snails 

 is not correctly placed in Pyramidida. They do not belong in the 

 EndodontidcB at all, but are Hdicidce, nearer Sonorella than any other 

 group, but constituting a new genus which will be suitably defined 

 in the near future. 



In South Australia there is a group of snails called Glyptorhagada, 

 some species of which resemble P. elrodi remarkably in form, sculp- 

 ture and color. They belong, however, to a different sub-family of 

 Helicida>,, and the resemblance, as in the case of E. circumcarinata, 

 is a case of convergent evolution. 



